1. Field of the Invention
This invention relates to reprocessing of solutions of acid and oxidants and more particularly to a double distillation reprocessing of a waste oxidant solution comprising sulfuric acid (H.sub.2 SO.sub.4) and ozone (O.sub.3) to obtain semiconductor grade sulfuric acid and ozone.
2. Prior Art
The semiconductor manufacturing industry uses oxidant solutions such as H.sub.2 SO.sub.4 or H.sub.2 SO.sub.4 in combination with an oxidant such as hydrogen peroxide (H.sub.2 O.sub.2), ammonium persulfate ((NH.sub.4).sub.2 S.sub.2 O.sub.8), or peroxydisulfuric acid (PDSA) (H.sub.2 S.sub.2 O.sub.8). These combinations of H.sub.2 SO.sub.4 and an oxidant are often referred to as "piranha acid." In the semiconductor integrated circuit manufacturing industry, these combinations are used at a semiconductor processing station to clean wafers and strip organic material such as photoresist. The used acid ("spent acid") is usually discarded after the processing operation.
Environmental concerns, economics and improved processing yields often make reprocessing of the spent acid a compelling option. However, reprocessing such acid to a semiconductor grade standard, with very low impurities and particulates, particularly on site at a semiconductor processing station, is a difficult task requiring sophisticated equipment, such as the reprocessor disclosed in co-pending U.S. patent application Ser. No. 07/231,849, filed Aug. 12, 1988, U.S. Pat. No. 4,980,032. Another approach for a sulfuric acid reprocessor is disclosed in U.S. Pat. No. 4,828,660. An on-site reprocessor must regenerate and recycle sulfuric acid from the combination of sulfuric acid and an oxidant if the oxidant is used in the processing station. Any reprocessor for this purpose will also need to supply this oxidant.
In the solutions containing an oxidant in addition to H.sub.2 SO.sub.4, the stripping or cleaning operation can be carried out at a lower temperature than when H.sub.2 SO.sub.4 alone is used. The lower stripping temperature has the advantage of reducing the amount of corrosive vapors given off at the point of use, in this case at a semiconductor processing station. In addition, the oxidant removes carbon from the solution which could redeposit on the wafer by reacting with carbon to form carbon dioxide (CO.sub.2).
Because of the advantages of using an oxidant solution, it is desirable for the reprocessing system to provide to the semiconductor processing station, in addition to ultrapure H.sub.2 SO.sub.4, some type of oxidant. It is well known to add ultrapure PDSA to the H.sub.2 SO.sub.4 in a H.sub.2 SO.sub.4 reprocessing system to provide an ultrapure oxidant solution comprising H.sub.2 SO.sub.4 and PDSA to the semiconductor processing station. For example, see U.S. Pat. No. 4,828,660 (wherein ultrapure PDSA is generated from an ultrapure stream of H.sub.2 SO.sub.4 directly from the H.sub.2 SO.sub.4 reprocessor). When adding PDSA to H.sub.2 SO.sub.4, a substantial amount of water is added to the oxidant solution due to the nature of PDSA manufacture. This water in the oxidant solution places a large load on the reprocessor since the water must be removed from the waste oxidant solution.
In addition to the oxidants named above, ozone has been found to be a good oxidant for use in combination with H.sub.2 SO.sub.4 for cleaning and stripping operations in semiconductor integrated circuit manufacture. See Japanese patent application publication No. 57-180132. The addition of ozone to the H.sub.2 SO.sub.4 does not involve the addition of any substantial water to the solution. However, ozone does not remain soluble in sulfuric acid which is heated (even at the lower stripping temperatures which are usually used) and therefore it is difficult to maintain a sufficient zone activity in the sulfuric acid bath used in semiconductor processing stations while at the same time keeping the concentration of ozone low in the surrounding area. These problems have complicated the use of ozone as an oxidant for water cleaning and so reprocessors have not been used with ozone as an oxidant. There are several disadvantages of using ozone in an oxidant solution that is to be reclaimed in a reprocessor. It is known to attack organic compounds, and could threaten the reliability of the reprocessor by corroding any organic components if not removed prior to reprocessing. Because ozone is very chemically active, it may interfere with any analytical measurements performed by the reprocessor. In addition, ozone is known to be toxic, so its concentration must be kept at a low level in the surrounding work area.